Eur. J. Entomol. 107: 377–392, 2010 http://www.eje.cz/scripts/viewabstract.php?abstract=1549 ISSN 1210-5759 (print), 1802-8829 (online) Descriptions of larvae of Megadytes (Coleoptera: Dytiscidae: Dytiscinae): The subgenera Trifurcitus and Megadytes s. str., ground plan of chaetotaxy of the genus and phylogenetic analysis MARIANO C. MICHAT CONICET, Laboratory of Entomology, DBBE, FCEyN, UBA, Buenos Aires, Argentina; e-mail: [email protected] Key words. Diving beetles, Dytiscidae, Cybistrini, Megadytes, Trifurcitus, larva, chaetotaxy, ground plan, phylogenetic relationships Abstract. The three larval instars of Megadytes (M.) carcharias Griffini and M. (Trifurcitus) fallax (Aubé) are described and illus- trated in detail for the first time, with an emphasis on morphometry and chaetotaxy of the cephalic capsule, head appendages, legs, last abdominal segment and urogomphi. The ground plan of chaetotaxy of the genus Megadytes Sharp is described and illustrated based on three of the four recognised subgenera. First-instar larvae of Megadytes are characterised by the presence of a large number of additional sensilla on almost every part of the body. Primary chaetotaxy of the subgenera (Bifurcitus Brinck based on third instar) is very similar, with few differences including (1) shape of the setae on the anterior margin of the frontoclypeus; (2) presence or absence of a ring of multi-branched setae on distal third of mandible; and (3) number of setae on the urogomphus. A cladistic analysis of Dytiscidae, based on 169 larval characters and 34 taxa, indicates that: (1) Trifurcitus Brinck deserves generic status; (2) Cybistrini are not closely related to Hydroporinae; (3) the absence of a galea in Cybistrini is a secondary loss independent of that in Hydroporinae; (4) Cybistrini are well supported by many characters (including several aspects of first-instar chaetotaxy). INTRODUCTION three the number of subgenera for which the first-instar A previous study (Michat, 2006) of the larval mor- larvae are known. A review of previous knowledge on the phology of the diving-beetle genus Megadytes Sharp larvae of Megadytes can be found in Michat (2006). Also, (Dytiscinae: Cybistrini) tested the hypothesis of a mono- descriptions of the mature larvae of M. giganteus phyletic origin of this genus and provided a key for the (Laporte) and M. latus (Fabricius) were presented by Fer- identification of the subgenera. At that time, however, the reira Jr. (2000) and Ferreira Jr. et al. (2006), respectively. first instar of only one species [M. (Paramegadytes) Based on the detailed generic description of the larvae of glaucus (Brullé)] was available for study, which pre- Megadytes presented in Michat (2006), I present here vented a broad analysis of the primary chaetotaxy. In par- diagnosis, descriptions and illustrations of the three larval ticular, any attempt to establish a ground plan for the instars of M. (M.) carcharias Griffini and M. (T.) fallax chaetotaxy of the genus was deemed to be premature. (Aubé). Also, the ground plan of chaetotaxy of the genus Members of the tribe Cybistrini are particularly inter- is described based on three of the four subgenera, and esting from a chaetotaxic perspective because their larvae interesting features are compared and discussed. have many additional sensilla (Nilsson, 1988; Michat, The finding of the first-instar larvae of members of Tri- 2006) which, to a certain degree, obscure the pattern of furcitus and Megadytes s. str. is also interesting because it ancestral sensilla that is commonly observed in members allows a reanalysis of previous results (Michat, 2006), of other dytiscid subfamilies and tribes of Dytiscinae. As which were based only on the mature larva, but now can demonstrated recently, primary chaetotaxy is an important also include characters of first and second instars. Though source of characters both for phylogenetic and diagnostic the monophyly of Dytiscinae, including Cybistrini, is well purposes (Alarie et al., 2007, 2009; Michat, 2006, 2008; supported by both adult (Miller, 2000, 2001, 2003; Miller Michat & Alarie, 2009; Michat & Archangelsky, 2009; et al., 2007) and larval (Michat, 2006) characters, recent Michat & Torres, 2006, 2008, 2009; Michat et al., 2007). studies based on DNA sequence data controversially indi- The development of a system of nomenclature for the pri- cate a position of Cybistrini outside the Dytiscinae, more mary sensilla of first-instar larvae of Dytiscidae resulted closely related to Hydroporinae and Laccophilinae in great progress in the exploration of an extensive set of (Ribera et al., 2008; see also Miller, 2003). On the other characters that is phylogenetically and diagnostically very hand, previous results (Michat, 2006) indicate that Mega- useful. dytes is not monophyletic, with members of the subgenus In this contribution the first-instar larvae of species Trifurcitus placed outside this genus and characterised by belonging to other two subgenera (Trifurcitus Brinck and a subtriangular head and evenly curved mandibles, that Megadytes s. str.) are described, which together with the lack a ring of multi-branched setae on the distal third, previously described first instar of Paramegadytes which contradicts the results of a previous study based on Trémouilles & Bachmann (Michat, 2006) increase to adult and molecular data (Miller et al., 2007). In the light 377 of this evidence, the primary objectives of the phyloge- Olympus CX31 compound microscope equipped with a camera netic analysis presented here are: (1) to re-evaluate, using lucida. Drawings were scanned and digitally edited. The mate- a larger larval data set, if the hypotheses of monophyly rial is held in the larval collection of M.C. Michat (Laboratory for the Cybistrini but not for Megadytes are corroborated, of Entomology, Buenos Aires University, Argentina). and (2) to test, based on a broader sampling of larval Morphometric and chaetotaxic analyses characters and taxa, the phylogenetic relationships of the The methods and terms used in the present paper follow those Cybistrini within Dytiscidae. employed in a previous paper on the larval morphology and chaetotaxy of Megadytes (Michat, 2006). The reader is also MATERIAL AND METHODS referred to Nilsson (1988) and Michat & Alarie (2009) for a Specimens were cleared in lactic acid, dissected and mounted complete list and additional explanations of the terms used in on glass slides in polyvinyl-lacto-glycerol. Observations (at the present study. magnifications up to 1,000×) and drawings were made using an TABLE 1. Taxa coded for cladistic analysis. Taxon Species Source ASPIDYTIDAE Aspidytes niobe Ribera, Beutel, Balke & Vogler, 2002 Alarie & Bilton, 2005 DYTISCIDAE AGABINAE Agabini Leuronectes curtulus Régimbart, 1899 – COLYMBETINAE Colymbetini Rhantus signatus (Fabricius, 1775) – COPELATINAE Copelatini Copelatus longicornis Sharp, 1882 – COPTOTOMINAE Coptotomini Coptotomus interrogatus (Fabricius, 1801) Barman, 2004 DYTISCINAE Aciliini Thermonectus succinctus (Aubé, 1838) – Aubehydrini Notaticus fasciatus Zimmermann, 1928 – Cybistrini Cybister lateralimarginalis (De Geer, 1774) Fiori, 1949; De Marzo, 1979 Megadytes (B.) giganteus (Laporte, 1835) Ferreira Jr., 1993 Megadytes (B.) magnus Trémouilles & Bachmann, 1980 – Megadytes (M.) carcharias Griffini, 1895 – Megadytes (M.) marginithorax (Perty, 1830) Ferreira Jr., 1995 Megadytes (P.) australis (Germain, 1854) – Megadytes (P.) glaucus (Brullé, 1838) – Megadytes (T.) fallax (Aubé, 1838) – Megadytes (T.) robustus (Aubé, 1838) – Onychohydrus scutellaris (Germar, 1848) Watts, 1964 Dytiscini Dytiscus harrisii Kirby, 1837 – Eretini Eretes australis (Erichson, 1842) Miller, 2002 Hydaticini Hydaticus tuyuensis Trémouilles, 1996 – Hyderodini Hyderodes shuckardi Hope, 1838 Bertrand, 1934; Watts, 1964 HYDROPORINAE Bidessini Anodocheilus maculatus Babington, 1841 – Hydroporini Canthyporus kenyensis Bilardo & Sanfilippo, 1979 Shaverdo & Alarie, 2006 Hydroporus columbianus Fall, 1923 Alarie & Harper, 1990; Alarie et al., 1990a; Alarie, 1991a,b Laccornellus lugubris (Aubé, 1838) – Hydrovatini Hydrovatus caraibus Sharp, 1882 – Hygrotini Hygrotus sayi J. Balfour-Browne, 1944 Alarie & Harper, 1990; Alarie et al., 1990a,b; Alarie, 1991a Hyphydrini Desmopachria concolor Sharp, 1882 – Laccornini Laccornis latens (Fall, 1937) Alarie & Harper, 1990; Alarie et al., 1990a; Alarie, 1989, 1991a Methlini Celina parallela (Babington, 1841) – Vatellini Vatellus haagi Wehncke, 1876 – LACCOPHILINAE Laccophilini Laccophilus obliquatus Régimbart, 1889 – LANCETINAE Lancetini Lancetes marginatus (Steinheil, 1869) – MATINAE Matini Matus bicarinatus (Say, 1823) Alarie et al., 2001 378 Phylogenetic analysis The phylogenetic relationships of members of the tribe Cybis- trini with other dytiscid groups were analyzed cladistically using the program TNT (Goloboff et al., 2008) and the character set provided by the larval morphology and chaetotaxy. A broad range of taxa was included, comprising members of nine of the 10 recognised dytiscid subfamilies (Nilsson, 2001) (Table 1). The subfamily Hydrodytinae was not included because their larvae are unknown. Data for most species were scored directly from observations on specimens and data for the remaining spe- cies from the literature. In order to avoid the problem of com- paring different semaphoronts (see Meier & Lim, 2009), in the codification of each character the same instars were compared, based on the fact that Dytiscidae larvae pass through three instars distinguishable by